Field of the Invention
This invention pertains to metering, collecting, filtering and distributing fluids. More specifically, the present invention relates to improved apparatus and improved methods for filtering and applying coating fluids onto substrates. The invention is useful for casting embossed sheeting, and in fluid application dies. Still further, the invention pertains to improved fluid filtration methods, apparatus, elements and media, and to the collection of mist generated in high speed liquid film splitting processes.
Background Information
A. Coating Technology—Fluid Metering
In processing fluids in general, it is often required to precisely control the flow rate or the distribution of local flow rates along a length. Coating is an example. Coating is the process of replacing gas contacting a substrate, usually a solid substrate such as a web, by a layer of fluid. In coating science there is the need to produce a controlled and commonly uniform distribution of local flow rates along a line. Generally, this is produced by forced flow through a slot and having it exit from a slot orifice of a coating die.
Most fluid flow devices are made of metal. They are precisely machined to very exacting dimensions, and they are expensive to fabricate. This is especially true of die coating devices. Inexpensive fabrication techniques and environmentally friendly disposable parts are needed. Coating processes using inexpensive and disposable coating apparatus would create competitive advantages. However, such devices are not available. Even disposable replacement parts for die coating devices are not known.
In manufacturing economic disposal of process waste is essential for low cost production. Incineration of waste is useful for this. However, contaminated metal parts damage incinerators. Polymeric and organic parts are ideal for disposal by incineration.
In coating dies the purpose of internal flow passages is to distribute the fluid so as to produce a film of fluid exiting from the die through the slot orifice along a length. Usually, it is desired that the rate flow be of uniform along the length. When fluid is transferred from the die to a moving web, the down web uniformity is dominated by the consistency of the web speed and the fluid supply rate. The cross web uniformity is a function of the uniformity of fluid flow from point to point across the web width.
Existing coating dies have internally a cavity and a metering slot connected in series. The slot and cavity serve to distribute flow along a length. The solid structure of the die defines the width of fluid applied onto a substrate during coating. When the cavity is large, the slot height small, and the slot dimensions uniform, a uniform flow exiting from the slot may be achieved. Over the years much art and science has been devoted to improving the design and understanding of the internal flow distribution of coating dies. This is described by Robert B. Secor in chapter 10 of Liquid Film Coating; Kistler, S. F. and Schweizer, P. M., Editors; Chapman & Hall: London, 1997.
Slots have limited ability to distribute flow from a cavity uniformly along a discharge edge length. Additionally, it is even more difficult to feed fluid to a slot at a single point and discharge it uniformly along a discharge edge length. Generally, very small slot gaps are required. Improvements are needed.
Unfortunately, the uniformity of flow from a die slot even when it is fed uniformly at an inlet edge is dominated by the precision of the slot height. If the fluid flow is laminar and the fluid is Newtonian, a 10 percent variation in the height will result in a 30 percent variation in flow rate. If the fluid is shear thinning the deviation will be higher. Therefore, the metering slot must be very precisely machined at great expense. The consequence is that coating dies are very costly to build, and very labor intensive to clean and maintain. It is not economical to dispose of them after a single use. An improvement over this die slot technology is needed.
In the processing of fluids to produce coated products the fluids must be filtered to remove particles with sizes ranging from about the dry coating caliper to about the wet coating caliper. These will cause visible or functional defects. Most particularly, particles with sizes near or larger than the hydraulic diameter of the flow passages must be removed to avoid severe disruption of the uniformity of the coating distribution. Conventionally, this is done as a step separate from the application of the fluid to a substrate, Since there are many sources of fluid contamination within the die, it is desirable to filter the fluid as it enters the die slot. However this is not generally possible. Simplified apparatus and methods are needed for the simultaneous final filtration and coating of fluids.
B. Arrays of Holes
Another aspect of this invention relates to fluid distribution using arrays of the fluid flow bores. Distribution dies using such are referred to as multi-orifice dies. Bores have been used in some instances to overcome some short comings of slots.
Flow distribution dies with continuous slots are expensive to manufacture and operate. The machining and setup costs for the slot fluid distribution dies are large. Maintaining a precisely uniform discharge slot is costly. In order to maintain uniformity of the discharge slot in the cross-web direction, dies have to be massive and require elaborate mountings to provide adequate structural support around the slot. Replacing the slot with drilled holes simplifies and reduces cost.
When dies are used for coating the “cross-web” direction is defined as the dimension across the width of a substrate translating with respect to the die. The web is typically a web of paper or polymer film. The “cross-web” direction is perpendicular to the direction of travel of the web with respect to the die. “Cross web” direction may be used to identify the orientation in the die, and the orientation of a plane intersecting the coating on a web or an extrudate or the web itself.
Dies with multiple orifices provided a less expensive alternative to continuous slot dies. Multiple orifice dies have a number of orifices that allow fluid to exit the die at a discharge face. Examples of multiple orifice dies are illustrated and described in U.S. Pat. Nos. 3,149,949, 4,774,109, 5,045,358 and 4,371,571, all of which are incorporated by reference in their entirety, herein.
Multiplicities of parallel drilled holes or bores, and porous media have been used in place of slots. All have deficiencies. McIntyre in U.S. Pat. No. 4,386,998 discloses a drilled hole coating die where the fluid is discharged along the length of a coating die through a line of cylindrical bores and exit orifices. The holes have a diameter on the order hundreds of microns. The drilled die is a useful design due to its simplicity.
While the multiple orifice die has utility, it also has substantial problems. First when a hole becomes plugged, an uncoated down web line occurs in the coating on the web. Once a hole is locally plugged there is no way to direct flow to the exit orifice of the plugged bore. Second, the small holes are difficult to clean. Thirdly, individual flow streams exiting from the orifices must be merged to form a continuous uninterrupted coating on the target substrate. There is a need to counter the drilled die plugging tendencies and improve the performance.
U.S. Pat. No. 7,591,903 by Maier et al. discloses multi-orifice dies used in coating. It discusses the use of a die with a face plate with a plurality of orifices and bores extending back into the die body. These bores are convey fluid from the die cavity to the die face, are independent of each other. They have no means of exchanging fluid between bores. When a bore is clogged at its entrance, no fluid flow exits from its discharge orifice at the die face.
C. Porous Media
Passing a fluid through a porous media to distribute it is known. U.S. Pat. No. 402,188 discloses a painting apparatus in which the paint flows through a porous piece of felt or sponge and is applied as a coating. The porous material serves to distribute across a width of substrate. U.S. Pat. No. 3,828,725 describes a curtain coater in which a bed of beads is placed in the supply cavity before the slot to increase the flow resistance into the slot and achieve lower flow rates. U.S. Pat. No. 3,365,325 discloses curtain coating using flow through one or more porous members to distribute flow into a free falling curtain for coating.
Seaver et al. in U.S. Pat. No. 5,702,527 disclose the use of a porous material of material compressed between two die plates to create a large pressure drop to produce uniform flow at low flow rates along the die width. The flow is within the sheet from one edge to another because of the confinement by the die plates.
The uniformity of flow from a distribution die with a cavity is limited by the uniformity of the media. The uniformity of flow from a discharge edge is quite limited when the media is fed from a point source or very small cavity in the die. Improvements are needed.
The uniformity of conventional commercially available porous materials is also deficient. They are not adequately uniform from point to point, and the individual pores are not uniformly positioned and sized. Improvements are needed.
Pores are small in materials used in porous media. Pore sizes generally range from submicrons to a hundred microns. Porous media material is a mixture of a solid framework and pores. A porous sheet is considered to have a length and width much larger than the pore size and a thickness many times greater than the pore size. The pores are distributed throughout the material.
Usually both the solid matrix and the pore network (the void volume) are assumed to be continuous. However in reality in known commercial materials, the pores are not all continuous and the void volumes in all areas of the material are not uniform. Some pores are dead ended, and some are totally isolated from the other pores. This is a failing of known porous media. Examples of deficient sheets are sheets formed from metal, ceramic, and plastic granular material.
Conventional porous materials have some portion of their void volume that is non-functional with respect to fluid flow. This results in restricted transport of fluid. In addition to these problems, the porous sheets described in the Seaver et al. patent and other porous materials are deficient in another way for precisely distributing fluids along a line of discharge. When examined closely, it is found that known porous sheets do not have uniform flow properties from point to point. In contrast, precision coating die slots have uniform flow resistances per unit length except at edges.
Known porous materials are fabricated from collections of fibers or particles placed together to form solid structures. Other porous materials are formed by mixing two or more different materials together. An example is the mixing of a gas with a liquid resin followed by the solidification of the resin. The individual fibers or particles always have a distribution of sizes, and therefore the sheet forming processes are not consistent. Bubbles in porous foams are not uniform in size and their locations relative to their neighbors are variable. The consequence is that the resulting porosity of these materials is not uniform. The resulting flow resistance of these sheets is not uniform along any line. Improvements are needed.
Sheets of porous media do not confine fluid flow to be solely within the plane of the sheet. The media allows flow in all directions including unfortunately perpendicularly out from and through their top and bottom sheet surfaces.
Sheets of known porous media have deficiencies in distributing flow and they do not perform well as filters. When flow is from and inlet edge to an outlet edge the inlet edge traps contaminants and disrupts flow uniformity. This adds to the non-uniformity cause by their base structure. Improvements in both functions individually and the simultaneous distribution and filtering are needed.
D. Filtering with Porous Media
One type of fluid filter is a cartridge-type filter with a replaceable filter element typically mounted on a core and placed into a filtration system. Other devices and filter media employing granular material structures and sheets are commercially available. Replaceable porous filters have pores sized to prevent contaminants and/or particles (hereinafter “contaminants” for the sake of convenience and without intent to limit) from passing through the filter, while allowing fluid passage. Contaminants typically become trapped on filter surfaces requiring the filters to be replaced on a regular basis. Example granular material structures include the use of metals, ceramics, plastics, sand and other like materials assembled so that the collections of grains form the porous media. Usually the grains are molded into a functional shape and sintered to form a block of filter media. Metal grains are commonly used in high pressure filter applications.
Cartridge-type pleated filters are cylindrical elements having an open longitudinal center bore with radially-outwardly extending, longitudinally folded portions or pleats. A plurality of pleats is commonly arranged around a tubular core defining a cylinder element. When viewed in a transverse cross-section, the pleats typically extend radially outward from the core toward the outer periphery of the filter. A drawback of standard pleated designs is that, because of standardization, it is difficult to increase the size of the usable filter area beyond that allowed by the conventional dimensions of the filter container. The filter capacity and effectiveness are limited by the surface area of the pleated cartridge design approach. Additionally the mechanical strength of the pleats is limited. Above a limiting pressure gradient across the filter the cartridge will fracture or collapse.
Attempts have been made to modify the pleat design in order to increase the surface area. For example, attempts have been made to modify the length at which a pleat extends from the center core toward the periphery of the cartridge. Clendenning et al. in U.S. Pat. No. 7,125,490 disclose forming pleats that are radially curved rather than having pleats that extend linearly from the core of the cartridge. The increase in the length of each radially curved pleat is intended to result in an increased surface area of the filter.
Plain, thick walled porous cylinders of sintered grains are known and used as substitutes for the pleated geometry to gain strength. However, the surface area for filtration is restricted to the circumference times the cylinder length. While this geometry is physically strong, the filtration area is limited. Attempts to improve these filters include molding of surface areas or removing material to obtain complex surface shapes to increase the exposed surface area. Omitting or removing material however diminishes the strength.
Previously, to reduce the pressure drop across the filter and improve filtration life, sintered metal, porous plastic and ceramic filters having extended filter surface areas have been designed. Such extended area filters include cylindrical or conical cavities in the filter's surface. Examples include Mott's U.S. Pat. No. 3,570,059 and Bergstrom's, U.S. Pat. Nos. 3,746,642 and 3,788,486. Such filters offer extended filter inlet side surface area. Often a method of providing extended surface area involves deforming the inlet surface by molding deep large pits in it. The scale of such inlet surface modification is on the order of 5 millimeters to many centimeters.
U.S. Pat. No. D618,761 illustrates a geometry where the structure of the block of filter media is highly modified to achieve increased surface area.
Haldopoulos et al. in U.S. Patent Application Publication 20080296238 disclose the use of molded, highly intricate, sintered porous plate structures in assemblages as replacements for pleated cartridges. To overcome the mechanical strength limitation of the long pleated cartridge filters they have replaced the conventional long pleated single element with a multitude of very short stacked plate elements. Each plate element, filter plate is comprised of a planar base portion having an outer peripheral edge, a top surface, and a bottom surface. A convoluted ridge wall extends from the top surface of the base portion and has a ridge outer side surface, a ridge inner side surface, and a top ridge surface. This ridge is in essence a short pleated subassembly which is attached to and strengthened by the base portion. In this manner they describe using the surface area advantage of pleating while providing means reinforcing the mechanical strength of the pleat.
In all cases the increase in surface area is obtained by omitting or removing large volumes of the base sintered porous media. This reduces the overall strength of the element lowering its fracture resistance or collapse strength. The total volume occupied by a pleated element is much larger than the actual volume occupied by the porous material itself. The lack of material completely filling the total volume very significantly diminishes the mechanical strength.
In general, if a sintered filtration media element is limited to the confines of a fixed volume, the strength of this element is maximized when the media totally fills the volume. Any removal or omission of the media from parts of the volume to increase surface area or enhance flow diminishes the strength. This is a basic problem with past designs of filter elements.
What is lacking are internal pore structures and designs to allow better filtration, allow more complete utilization of pores, and to allow reduced flow resistance.
The nature of the granular make-up of the sintered a metal filter and its counterparts in other materials is a severe limitation. There is no way to modify and control the individual flow and particle capture properties of the individual pores within the media. The assemblage of grains creates a random collection of pore locations, sizes and interconnections. A method is needed to create porous structure where a multiplicity of the pore locations, sizes, interconnections and properties maybe individually designed and manipulated to produce improve flows and filtering.
E. Disposable Fluid Distribution Components
Most fluid flow devices and in particular coating devices are made of metal. Although they may be precisely machined to very exacting dimensions, they are expensive to fabricate. This is especially true of slot die coating devices. Inexpensive fabrication techniques and environmentally friendly disposable parts are needed. Coating processes using inexpensive and disposable coating apparatus would create a competitive advantage. However, such devices are not available. Even economically disposable replacement parts for die coating devices are not known.
Fluid transport may be characterized based on the mechanism that causes flow within the device. When fluid transport is the result of a controllable force or gravity acting on the fluid, the fluid movement is considered “forced”. An example is the flow resulting from an applied pressure force. Such a pressure may be generated as a result of the active forces created by displacement of a fluid using a pump.
In processing, precise control the flow rate or the distribution of local flow rates along a length is required. Coating is such a process. Coating consists of replacing gas contacting a substrate, usually a solid substrate such as a web, by a layer of fluid. Generally, a uniform distribution of local flow rates along a discharge slot orifice is desired. This is produced by forced flow through a slot of a die coater device.
1. Fluid Metering by Coating Slots
In coating dies the purpose of internal flow passages is to distribute the fluid so as to produce a film of fluid exiting through a slot orifice along a length. Commonly, it is desired that the rate flow be of uniform along the length. When fluid is transferred from the die to a moving web, the down web uniformity is dominantly controlled by the consistency of the web speed and the fluid supply rate. The cross web uniformity is a function of the uniformity of fluid flow from point to point across the web width.
Prior coating techniques are illustrated in FIGS. 68 and 46. FIG. 68 is a cross sectional schematic showing the internal flow passages in a coating die. Coating dies have internally a cavity 1000 and a metering slot 1002 connected in series. These are confined by a top die plate 1005 and a bottom die plate 1006. The slot and cavity serve to distribute flow along a length. The slot extends along the length of the die to positions near its ends. The flow exits from a slot orifice 1003 which is bounded by two die lips 1007 and 1008 at the distal ends for plates 1005 and 1006. Solid structure of the die at its ends defines the width of fluid applied onto a substrate. FIG. 46 is an isometric view of the internal flow passages. The cavity 501 takes the fluid inflow from a feed point illustrated by the arrow 503 and distributes it along the internal entrance of the slot 502. When the cavity is large, the slot height small, and the slot dimensions uniform, constant flow exiting from the slot may be achieved along the length of the die slot. This is illustrated by the arrows 504. Over the years much art and science has been devoted to improving the design and understanding of the internal flow distribution of coating dies. This is described by Robert B. Secor in Liquid Film Coating; Kistler, S. F. and Schweizer, P. M., Editors; Chapman & Hall: London, 1997; chapter 10.
Unfortunately, the uniformity of flow from a die slot is dominated by the precision (the uniformity) of the slot height. If the fluid flow is laminar and the fluid is Newtonian, a 10 percent variation in the height will result in a 30 percent variation in flow rate. If the fluid is shear thinning the deviation will be higher. Therefore, the metering slot must be very precisely machined at great expense. The consequence is that coating dies are very costly, and they are labor intensive to clean and maintain. It is not economical to dispose of them after a single use. An improvement is needed.
2. Arrays of Holes
As described, the function of a metering slot is usually to provide prescribed flow along a line where it exits the die. Other methods of achieving this include the use of a multiplicity of parallel drilled holes and the use of porous media. Both have deficiencies.
McIntyre in U.S. Pat. No. 4,386,998 discloses a coating die where the fluid is discharged across the width of a coating die through a line of cylindrical drilled holes. The holes have a diameter on the order hundreds of microns. While this has some utility, it also has substantial problems. First when a hole becomes plugged, an uncoated down web line occurs in the coating on the web. Once a hole is locally plugged there is no way to divert flow to the exiting orifice of the plugged hole. Second, the small holes are difficult to clean. This contrasts with the ease of cleaning a die slot. Metering slots are positioned at the junction of two die plates which may be disassembled for intense cleaning of the slot surfaces. Thorough cleaning a great many small holes is labor intensive.
Insley et al. in U.S. Pat. No. 6,290,685 describes the use of micro-replicated parallel flow channels to create a fluid distribution sheet, but these suffer from the same problems as the holes of the McIntyre patent.
3. Porous Media
Although porous media are widely available and often consider disposable, it is deficient. The problems and deficiencies of commercial porous media are discussed above.
4. Blade Coating
Blade coating is a method of coating in the paper industry. Blade coating is a method by which coating is applied to base paper. The coater consists of a large back-up roll around which the paper passes and a steel blade which bears against the paper. Excess coating fluid is applied to the paper and the excess is scraped off by the blade. Eklund et al. in U.S. Pat. No. 4,945,855 describes a blade coater where fresh, excess fluid is deposited onto the paper directly behind the blade. A problem with this method is that no means exists that allows the coating weight applied to be directly controlled by metering the flow rate and where no excess is used.
5. Coating Die Precision Lips
Coatings of many types are applied with slot orifice dies. The die is a means of spreading a premetered amount of fluid onto the width of a substrate. The fluid is provided from a hose or pipe and is distributed across the substrate by the die. The fluid exits through a slot orifice at die lips positioned very close to the substrate. Fluid is transferred from the slot to the substrate to produce a coating on the substrate.
The die plate areas around the slot orifice opening are referred to as lips. The edges of the slot orifice on the lips are a critical region for defect free coatings. Any nick or protrusion at the orifice generates defects in the coating even if the average flow through the slot is uniform—even if the rest of the slot is perfect. Unfortunately, the orifice edges and the lips are easily damaged. Incidental contact with objects frequently occurs necessitating replacing these expensive lips and the die plates which contain them. Improvements in the durability of die lips are needed. Inexpensive lips are needed.
Unfortunately, the orifice edges in the lips are easily damaged. Incidental contact with objects frequently occurs necessitating replacing this expensive lips and the die plates which contain them. Additionally, the metal die parts when damaged are contaminated by the coating materials which are often hazardous materials. Incineration of the parts may be required to destroy the hazardous materials. However, the metal parts still remain and increase the cost of the disposal. Improvements in the durability of die lips are needed as well as improvements in the die lip materials to facilitate disposal.
Coating dies are made of tool steel. This is used because the steel may be machined and ground with very high precision. Precision die slots are thought necessary for generating uniform flow from the slot orifice. Precision ground die lips are required for accurate positioning of the die lips very close to the substrate during coating. Flatness, straightness and positioning precisions of plus or minus 2 micrometers are often required with these tool steels pieces. Therefore, dies are currently are very expensive to manufacture.
In summary, an apparatus that eliminates the need for precisely machined metal die lips and slots is desired.
a. Cast Coating Die Lips
During cast coating, the dies are bought into very close proximity to the substrate and/or coating rolls. Unfortunately, positioning of the dies is subject to human error and mechanical breakdowns, and the result is often clashing of the die lips with the substrate or coating rolls. Touching the lips to the substrate being coated will damage it. If the substrate is a web, cutting or breaking the web may occur. If the substrate is a metal embossing belt, the embossing pattern will be destroyed. If the die is used in combination with a roll, clashing the lips and the roll will damage the surfaces. If the roll has a precisely patterned surface it must be replaced. Clashing causes expensive upsets to the manufacturing process for coated products.
A means to minimize the costs of clashing die lips is desired. This is particularly useful in the production of embossed sheeting and pressure sensitive adhesive coated products.
Die lips may be integral to the plates making up the die, or they may be separate items attached to the die plates. In any case, they must be precisely machined and ground so that they may be positioned very accurately to create uniform gaps between the lip and the substrate during coating. The uniformity of these gaps across the width of the substrate determines the uniformity of the coating applied. A one percent variation of the gap results in a one percent or greater variation in the cross web coating uniformity. The expense of manufacturing steel die lips with the accuracy necessary for precision coating is large.
b. Casting Manufacture of Embossed Sheeting
Embossed or patterned sheeting is generally characterized as having a smooth side and a textured side. It is widely used for many purposes. Optically functional sheeting is one class that highly important commercially. Examples include cube corner retro-reflective sheeting as exemplified in U.S. Pat. No. 6,884,371 to Smith. Cube corner retroreflective sheeting typically comprises a thin transparent layer having a substantially planar front surface and a rear structured surface comprising a plurality of geometric structures. The process of making cube corner sheets is complex and expensive.
Cube corner retroreflective sheeting is only one example of many useful types of functional embossed sheeting. Other examples include the production of surface 3-dimensional structures (textures) of both large and small dimensions. Macro and micro-structures on a surface serve many purposes ranging from modification of optical properties, frictional properties, fluid flow interactions, and interaction with electromagnetic energy.
A significant portion of the manufacturing expense is associated with the creation of masters, molds and tooling used for creation of the embossed sheeting. Sheeting is manufactured by casting a thin layer of fluid polymeric resin onto a roll, belt, web or flat mold. The mold contains on its surface the negative of the desired sheeting surface texture. The casting process uses precision coating dies to apply the liquid onto these textured surfaces. After solidification of the resin, an embossed sheet is striped from the casting surface. The mold is reused as many times as possible to reduce the mold investment cost per unit of product. To achieve precision and durability the textured mold surface is commonly constructed from metal.
It is common to cast this liquid layer of resin onto the mold surface using a slot orifice coating die. Slot die heads are known as a means of achieving a precise and smooth liquid film on a web, belt, mold or master. These dies have a slot orifice from which flows id directed onto the target surface. The slot exit is defined by a pair of lips. One of the lips may be adjustable in directions substantially perpendicular to the slot axis. This provides adjustment of the slot opening for tuning the uniformity of the fluid flow from the slot.
The die lips extend from the slot upstream and downstream relative to the direction of mold or web movement. The downstream lip serves as a smoothing lip for transferring and forcing fluid coating onto the mold. In order to achieve proper transfer and surface filling, the smoothing lip must be positioned extremely close to the mold surface. To achieve a useful smooth continuous coating on the mold, the die lip must be positioned with a clearance equal to one to two times desired sheet thickness. If the sheet has a wet caliper of 50 micrometers before solidification, the lip will need to be within 50 to 100 micrometers from the mold surface.
The operational positioning of the slot die very close to the mold surface is difficult and demands constant attention. Unfortunately, problems often occur, and the die may contact the mold. Any contact or clashing with the metal die lip damages the mold. This requires replacement of the expense mold. The metal die lip is also damaged by clashing, and it also requires replacement.
c. Casting Manufacture of PSA Coatings
The casting of unsupported pressure sensitive adhesive (PSA) onto rolls or belts using slot coating dies has problems similar to casting sheeting.
Problems in PSA casting are exemplified by the need for a method to coat a hotmelt adhesive onto an open polypropylene non-woven web without penetrating into it. The challenge is that the molten hotmelt adhesive has a low viscosity at its application temperature. Coating it with a slot die forces the adhesive into and through the non-woven. A method to avoid this problem is to cast the adhesive onto a cooled transfer drum or belt and then laminate solidified adhesive to the non-woven web.
Coating onto a steel transfer drum or belt is fraught with problems. It generates scrap and roll surface maintenance down time. To coat a thin layer of PSA onto a chilled roll requires the surface must be coated with a silicone release coating to allow removal afterward solidification.
The slot die steel lip must be brought very precisely into close proximity to the roll surface. When a coating is produced, it is non-uniform and of poor quality. This is a result of the limitations on dimensional tolerances when machining the die and the roll. Tolerances are further upset by variations in the release coating on the roll. There is great risk of mechanical contact between the roll and the metal lip. This destroys the roll release coatings and metal surfaces. The thin coatings required for PSA tapes are difficult to produce at low scrap rates using transfer drums.
d. Cast Coating Improvements
Both the production of embossed sheeting, and the casting of free PSA films have the common problem of damage to critical surfaces when using a slot die. Improvements in coating onto reusable surfaces using casting dies is a need of industry.
Lippert in U.S. Pat. No. 5,067,432 describes an improved slot die useful for casting a coating onto a web or mold surface. The improvement comprises a means of removably attaching the lips to the coating die. While this allows easy replacement of die lips damaged by clashing, it does not prevent the damage. Improvements are desired in the casting process to minimize die cost, to minimize die maintenance, to minimize mold maintenance, and to maximize component life.
e. Waste Management with Dies
As previously noted, precision coating with a die requires uniform flow across the width of the substrate. Uniform flow requires a uniform slot height along its length and across the width. Currently, this uniformity requires precision metal dies. Great skill and art is employed to machine grind and polish the die plates to the needed exacting tolerances. This makes die plates expensive to make and maintain. Disposal after a single use is not economical. Cleaning and reuse of the die is time consuming and labor intensive.
Labor saving improvements in the die maintenance are desired. Inexpensive coating dies which allow environmentally friendly disposal are needed. Methods of coating using inexpensive disposable coating apparatus are needed. Disposal by incineration is desired. Disposable devices or even disposable replacement parts are not known. These will have the greatest economic impact if they are manufactured by high volume processes.
f. Mist Collection
Multi-roll coaters are common for the production of solventless silicone coatings in the converting industry. These are described in chapter 2 of the Evolution of Coating, by George L. Booth published by Gorham International Inc., of Gorham, Me. in 1995. As process speeds increase, the generation of mists in the roll nips has become a problem. A mist is defined as a concentration of particles in a gas where the particles are generally liquid or solid. There is a special need for the elimination of mists from the process of silicone coating. Mist abatement offers the potential of lower operational costs, lower health hazards, reduced contamination of the process area, and elimination of unwanted redeposits on the coated product. Silicone contamination of surfaces modifies their surface properties and changes their functionality in many negative ways.
The fundamentals of the misting problem have been studied by Michael Sean Owens in his PhD studies at the University of Minnesota and are summarized in his PowerPoint™ Doctoral Defense presentation on Oct. 27, 2004. Potential solutions are suggested by his work. He shows that manipulation of process variables, formulation rheology, and formulation chemistry can produce process operating windows where misting is diminished. Unfortunately, increasing web line speed will at some point always create a misting problem. A limitation of Owens' work is that he did not study the ultra-low coating range common in silicone coating.
Anti-misting additives and formulations have been developed by the chemical suppliers. Examples of this approach include U.S. Pat. Nos. 5,698,655, 4,806,391, 6,057,033 and 6,511,714. It should be recognized that no additive or special formulation technology will likely have the flexibility to solve all silicone misting problems. These disclosures are helpful, but they do not give product developers the unrestrained freedom to optimize their formulations for other properties.
Misting is also common in the paper industry and in the high speed use of the coating improvement rolls described by Leonard et al. in U.S. Pat. Nos. 6,737,113, 7,311,780, 6,899,922, 6,855,374 and 6,579,574.
When mists are created on a coating process line, the traditional HVAC engineers have attempted to collect and dispose of them with air handling systems. In order to guard against the consequences of this mist, air ducts for mist capture are positioned around the coating station. The resulting ducting and baffles are objectionable. They obstruct access to the coater. They obstruct observation and monitoring of the process. They hinder cleaning of the coater. Additionally, large volumes of contaminated air are generated which are costly to process.
One of the disclosures of Owens is to use large diameter coater rolls. This is helpful because it decreases the rate of divergence of the roll surfaces emerging from the nip. A practical method of cutting the divergence in half at the transfer nip is suggested by this inventor. Using a lead-off idler roll to direct the web path on the outrunning side of the transfer nip can create the effect of an infinite backup roll diameter. This is accomplished by removing the web from the backup roll surface at the nip. The web moves forward in contact with the transfer roll. It then is removed tangentially from the transfer roll and passes on to the idler roll. Note that this approach may still fail at a sufficiently high speed. Additionally, the approach generally changes the dynamics of web handling especially with regard to elements of tension control, speed control, wrinkling abatement, and coating quality. Success is dependent on the silicone formulation, web type and web quality. Expert web handling advice is available from various consultants. Again this approach, like so many others, fails at a high speed.
All US patents and patent applications, and all other published documents mentioned anywhere in this application are incorporated by reference in their entirety.